1. Field of the Invention
The present invention relates generally to a strip dual mode filter utilized to filter microwaves in frequency bands ranging from an ultra high frequency (UHF) band to a super high frequency (SHF) band, and more particularly to a strip dual mode filter in which a resonance width of the microwaves is suitably adjusted. Also, the present invention relates to a dual mode multistage filter in which the strip dual mode filters are arranged in series.
2. Description of the Related Art
A half-wave length open end type of strip ring resonating filter has been generally utilized to filter microwaves ranging from the UHF band to the SHF band. Also, a one-wavelength type of strip ring resonating filter has been recently known. In the one-wavelength type of strip ring resonating filter, no open end to reflect the microwaves is required because a line length of the strip ring resonating filter is equivalent to one wavelength of the microwaves. Therefore, the microwaves are efficiently filtered because energy of the microwaves is not lost in the open end.
However, there are many drawbacks in the one-wavelength type of strip ring resonating filter. That is, it is difficult to manufacture a small-sized strip ring resonating filter because a central portion surrounded by the strip ring resonating filter is a dead space.
Therefore, a dual mode filter in which microwaves in two orthogonal modes are resonated and filtered has been recently proposed. The dual mode filter has not yet been put to practical use.
2-1 Previously Proposed Art
A first conventional strip dual mode filter is described.
FIG. 1 is a plan view of a strip dual mode filter functioning as a two-stage filter.
As shown in FIG. 1, a strip dual mode filter 11 conventionally utilized is provided with an input strip line 12 in which microwaves are transmitted, a one-wavelength type of strip ring resonator 13 electrically coupled to the input strip line in capacitive coupling, and an output strip line 14 electrically coupled to the strip rink resonator 13 in capacitive coupling.
The input strip line 12 is coupled to the strip ring resonator 13 through a gap capacitor 15, and the output strip line 14 is coupled to the strip ring resonator 13 through a gap capacitor 16. Also, the output strip line 14 is spaced 90 degrees (or a quarter of a wavelength of the microwaves) in electric length apart from the input strip line 12.
The strip ring resonator 13 has an open end stub 17 in which the microwaves are reflected. The open end stub 17 is spaced 135 degrees in the electric length apart from the input and output strip lines 12, 14.
In the above configuration, the action of the strip dual mode filter 11 is qualitatively described in a concept of travelling wave.
When a travelling wave is transmitted in the input strip line 12, electric field is induced in the gap capacitor 15. Therefore, the input strip line 12 is coupled to the strip ring resonator 13 in the capacitive coupling, so that a strong intensity of electric field is induced to a coupling point P1 of the strip ring resonator 13 adjacent to the input strip line 12. The electric field strongly induced is diffused into the strip ring resonator 13 as travelling waves. That is, one of the travelling waves is transmitted in a clockwise direction and another travelling wave is transmitted in a counterclockwise direction.
An action of the travelling wave transmitted in the counterclockwise direction is initially described.
When the travelling wave reaches a coupling point P2 of the strip ring resonator 13 adjacent to the output line 14, the phase of the travelling wave is shifted 90 degrees. Therefore, the intensity of the electric field at the coupling point P2 is minimized. Accordingly, the output strip line 14 is not coupled to the strip ring resonator 13 in the capacitive coupling.
Thereafter, when the travelling wave reaches the open end stub 17, the phase of the travelling wave is further shifted 135 degrees as compared with the phase of the travelling wave reaching the coupling point P2. Because the open end stub 17 is equivalent to a discontinuous portion of the strip ring resonator 13, a part of the travelling wave is reflected at the open end stub 17 to produce a reflected wave, and a remaining part of the travelling wave is not reflected at the open end stub 17 to produce a non-reflected wave.
The non-reflected wave is transmitted to the coupling point P1. In this case, because the phase of the non-reflected wave transmitted to the coupling point P1 is totally shifted 360 degrees as compared with that of the travelling wave transmitted from the input strip line 12 to the coupling point P1, the intensity of the electric field at the coupling point P1 is maximized. Therefore, the input strip line 12 is coupled to the strip ring resonator 13 so that a part of the non-reflected wave is returned to the input strip line 12. A remaining part of the non-reflected wave is again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring resonator 13 are resonated.
In contrast, the reflected wave is returned to the coupling point P2. In this case, the phase of the reflected wave at the coupling point P2 is further shifted 135 degrees as compared with that of the reflected wave at the open end stub 17. This is, the phase of the reflected wave at the coupling point P2 is totally shifted 360 degrees as compared with that of the travelling wave transferred from the input strip line 12 to the coupling point P1. Therefore, the intensity of the electric field at the coupling point P2 is maximized, so that the output strip line 12 is coupled to the strip ring resonator 13. As a result, a part of the reflected wave is transferred to the input strip line 12. A remaining part of the reflected wave is again circulated in the clockwise direction so that the microwaves transferred to the strip ring resonator 13 are resonated.
Next, the travelling wave transmitted in the clockwise direction is described.
A part of the travelling wave is reflected at the open end stub 17 to produce a reflected wave when the phase of the travelling wave is shifted 135 degrees. A non-reflected wave formed of a remaining part of the travelling wave reaches the coupling point P2. The phase of the non-reflected wave is totally shifted 270 degrees so that an intensity of the electric field induced by the non-reflected wave is minimized. Therefore, the non-reflected wave is not transferred to the output strip line 14. That is, a part of the non-reflected wave is transferred to the input strip line 12 in the same manner, and a remaining part of the non-reflected wave is again circulated in the clockwise direction so that the microwaves transferred to the strip ring resonator 13 are resonated.
In contrast, the reflected wave is return to the coupling point P1. In this case, because the phase of the reflected wave at the coupling point P1 is totally shifted 270 degrees, an intensity of the electric field induced by the reflected wave is minimized so that the reflected wave is not transferred to the input strip line 12. Thereafter, the reflected wave reaches the coupling point P2. In this case, because the phase of the reflected wave at the coupling point P2 is totally shifted 360 degrees, an intensity of the electric field induced by the reflected wave is maximized. Therefore, a part of the reflected wave is transferred to the output strip line 14, and a remaining part of the reflected wave is again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring resonator 13 are resonated.
Accordingly, because the microwaves can be resonated in the strip ring resonator 13 on condition that a wavelength of the microwaves equals the strip line length of the strip ring resonator 13, the strip dual mode filter 11 functions as a resonator and a filter.
Also, the microwaves transferred from the input strip line 12 are initially transmitted in the strip ring resonator 13 as the non-reflected waves, and the microwaves are again transmitted in the strip ring resonator 13 as the reflected waves shifted 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected wave and the reflected wave independently coexist in the strip ring resonator 13. Therefore, the strip dual mode filter 11 functions as a dual mode filter. That is, the function of the strip dual mode filter 11 is equivalent to a pair of a single mode filters arranged in series.
In addition, a ratio in the intensity of the reflected wave to the non-reflected wave is changed in proportional to the length of the open end stub 17 projected in a radial direction of the strip ring resonator 13. Therefore, the intensity of the reflected microwaves transferred to the output strip line 14 can be adjusted by trimming the open end stub 17.
The strip dual mode filter 11 is proposed by J. A. Curtis "International Microwave Symposium Digest", IEEE, page 443-446 (N-1), 1991.
2-2 Another Previously Proposed Art
Next, a conventional multistage filter is described.
FIG. 2A is a plan view of a conventional multistage filter in which two strip dual mode filters 11 are arranged in series.
As shown in FIG. 2A, a conventional multistage filter 21 consists of the strip dual mode filter 11a in a first stage, the strip dual mode filter 11b in a second stage, an inter-stage strip line 22 of which one end is coupled to a coupling point P3 spaced 90 degrees apart from the coupling point P1 of the strip dual mode filter 11a and another end is coupled to a coupling point P4 spaced 90 degrees apart from the coupling point P2 of the strip dual mode filter 11b, and a secondary inter-stage strip line 23 of which one end is coupled to a coupling point P5 spaced 180 degrees apart from the coupling point P1 of the strip dual mode filter 11a and another end is coupled to a coupling point P6 spaced 180 degrees apart from the coupling point P2 of the strip dual mode filter 11b.
In the above configuration, when microwaves are transferred to the coupling point P1 of the strip dual mode filter 11a, a greater part of the microwaves are reflected at the open end stub 17 of the strip dual mode filter 11a to produce reflected microwaves. Also, a remaining part of the microwaves are not reflected to produce non-reflected microwaves. Thereafter, the intensity of the electric field induced by the reflected microwaves is maximized at the coupling point P3 of the strip dual mode filter 11a. Therefore, the reflected microwaves are transferred to the strip dual mode filter 11b through the inter-stage strip line 22. Thereafter, the reflected microwaves are again reflected at the open end stub 17 of the strip dual mode filter 11b so that the intensity of the electric field at the coupling point P2 is maximized. Therefore, the reflected microwaves are transferred to the output strip line 14.
Also, the non-reflected microwaves are circulated in the strip dual mode filter 11a, and the intensity of the electric field induced by the non-reflected microwaves is maximized at the coupling point P5. Therefore, the non-reflected microwaves are transferred to the coupling point P6 of the strip dual mode filter 11b through the secondary inter-stage strip line 23. Thereafter, the non-reflected microwaves are circulated in the strip dual mode filter 11b, and the intensity of the electric field induced by the non-reflected microwaves is maximized at the coupling point P2. Therefore, the non-reflected microwaves are also transferred to the output strip line 14.
In this case, the strip dual mode filters 11a, 11b respectively function as a resonator and filter in dual modes for the reflected microwaves. Therefore, a resonance width of the reflected microwaves obtained in the output strip line 14 is narrow. In contrast, the strip dual mode filters 11a, 11b respectively function as a resonator and filter in a single mode for the non-reflected microwaves. Therefore, a resonance width of the non-reflected microwaves obtained in the output strip line 14 is wide.
Also, the phase of the reflected microwaves shifts by 90 degrees in the strip dual mode filter 11a as compared with that of the non-reflected microwaves, and the phase of the reflected microwaves additionally shifts by 90 degrees in the strip dual mode filter 11b as compared with that of the non-reflected microwaves. Therefore, the phase of the reflected microwaves totally shifts by 180 degrees as compared with that of the non-reflected microwaves.
In addition, the intensity of the reflected microwaves is greatly larger than that of the non-reflected microwaves.
Therefore, as shown in FIG. 2B, frequency characteristics of the reflected microwaves and the non-reflected microwaves are obtained. As a result, the reflected microwaves and the non-reflected are interfered with each other in the output strip line 14 to produce interfered microwaves. In this case, as shown in FIG. 2C, two notches (or two poles) are generated at both sides of a resonance frequency .omega..sub.o (or a central frequency) of the interfered microwaves.
As is well known, when a fundamental component of the microwaves is resonated and filtered in the multistage filter 21, a resonance width 2.DELTA..omega. of the fundamental component is greatly narrow. However, when an N-degree harmonic component of the microwaves is resonated and filtered in the multistage filter 21, a resonance width 2N.DELTA..omega. of the N-degree harmonic component becomes wide in proportion as the number N is increased.
Accordingly, the fundamental component of the microwaves and a few low-degree harmonic components of the microwaves can be steeply resonated and filtered in the multistage filter 21. Therefore, the multistage filter 21 can function as an elliptic filter in which the notches are deeply generated at both sides of the resonance frequency.
2-3 Problems to be Solved by the Invention
However, there are many drawbacks in the strip dual mode filter 11. That is, because a resonance width (or a full width at half maximum) is adjusted only by trimming the length of the open end stub 17, the resonance width cannot be enlarged. In other words, in cases where the width of the open end stub 17 in the circumferential direction is widened to enlarge the resonance width, the phase of the reflected wave reaching the output strip line 14 is undesirably shifted. As a result, the intensity of the microwaves transmitting through the output strip line 14 is lowered at a central wavelength (or a resonance frequency) of the microwaves resonated.
In addition, in cases where a plurality of strip dual mode filter 11 are arranged in series to manufacture a multistage filter, the resonance width of the multistage filter is furthermore narrowed. Accordingly, the multistage filter is not useful for practical use.
Also, there are many drawbacks in the multistage filter 21. That is, because the reflected microwaves are produced by only the open end stubs 17, the characteristic impedance of the multistage filter 21 cannot be suitably adjusted. Also, a resonance width in the filter 21 is narrowed so that the multistage filter 21 is not useful for practical use.